744 research outputs found
Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII
were inferred recently in the X-ray spectrum of the bursting neutron star EXO
0748-676. We place an upper limit on the stellar magnetic field based on the
iron lines. The oxygen absorption feature shows a multiple component profile
that is consistent with Zeeman splitting in a magnetic field of ~(1-2)x10^9
gauss, and for which the corresponding Zeeman components of the iron lines are
expected to be blended together. In other systems, a field strength >5x10^{10}
gauss could induce a blueshift of the line centroids that would counteract
gravitational redshift and complicate the derivation of constraints on the
equation of state of the neutron star.Comment: 5 pages, submitted to Phys. Rev. Let
Fermi-surface collapse and dynamical scaling near a quantum critical point
Quantum criticality arises when a macroscopic phase of matter undergoes a
continuous transformation at zero temperature. While the collective
fluctuations at quantum-critical points are being increasingly recognized as
playing an important role in a wide range of quantum materials, the nature of
the underlying quantum-critical excitations remains poorly understood. Here we
report in-depth measurements of the Hall effect in the heavy-fermion metal
YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid
crossover of the isothermal Hall coefficient clearly connected to the
quantum-critical point from a smooth background contribution; the latter exists
away from the quantum-critical point and is detectable through our studies only
over a wide range of magnetic field. Importantly, the width of the critical
crossover is proportional to temperature, which violates the predictions of
conventional theory and is instead consistent with an energy over temperature,
E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our
results provide evidence that the quantum-dynamical scaling and a critical
Kondo breakdown simultaneously operate in the same material. Correspondingly,
we infer that macroscopic scale-invariant fluctuations emerge from the
microscopic many-body excitations associated with a collapsing Fermi-surface.
This insight is expected to be relevant to the unconventional
finite-temperature behavior in a broad range of strongly correlated quantum
systems.Comment: 5 pages, plus supporting materia
Comment on "Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction in YbRh2Si2"
In Phys. Rev. Lett. 106, 137002 (2011), A. Hackl and M. Vojta have proposed
to explain the quantum critical behavior of YbRh2Si2 in terms of a
Zeeman-induced Lifshitz transition of an electronic band whose width is about 6
orders of magnitude smaller than that of conventional metals. Here, we note
that the ultra-narrowness of the proposed band, as well as the proposed
scenario per se, lead to properties which are qualitatively inconsistent with
the salient features observed in YbRh2Si2 near its quantum critical point.Comment: 3 page
Electrical resistivity ofYb(Rh1-xCox)2Si2 single crystals at low temperatures
We report low-temperature measurements of the electrical resistivity of
Yb(Rh1-xCox)2Si2 single crystals with 0 <= x <= 0.12. The isoelectronic
substitution of Co on the Rh site leads to a decrease of the unit cell volume
which stabilizes the antiferromagnetism. Consequently, the antiferromagnetic
transition temperature increases upon Co substitution. For x = 0.07 Co content
a subsequent low-temperature transition is observed in agreement with
susceptibility measurements and results on YbRh2Si2 under hydrostatic pressure.
Above the Neel transition the resistivity follows a non-Fermi liquid behavior
similar to that of YbRh2Si2.Comment: 4 pages, submitted to SCES0
Heavy Fermions and Quantum Phase Transitions
Quantum phase transitions arise in many-body systems due to competing
interactions that promote rivaling ground states. Recent years have seen the
identification of continuous quantum phase transitions, or quantum critical
points, in a host of antiferromagnetic heavy-fermion compounds. Studies of the
interplay between the various effects have revealed new classes of quantum
critical points, and are uncovering a plethora of new quantum phases. At the
same time, quantum criticality has provided fresh insights into the electronic,
magnetic, and superconducting properties of the heavy-fermion metals. We review
these developments, discuss the open issues, and outline some directions for
future research.Comment: review article, 26 pages, 4 figure
Field dependence of the thermopower of CeNiSn
Previously measured thermopower data of CeNiSn exhibit a significant sample
dependence and non-monotonous behavior in magnetic fields. In this paper we
demonstrate that the measured thermopower S(T) may contain a contribution from
the huge Nernst coefficient of the compound, even in moderate fields of 2 T. A
correction for this effect allows to determine the intrinsic field dependence
of S(T). The observed thermopower behavior can be understood from Zeeman
splitting of a V-shaped pseudogap in magnetic fields.Comment: 4 pages, accepted for Journal of Physics: Conference Series,
proceedings of LT 2
Anisotropic Hall Effect in Single Crystal Heavy Fermion YbAgGe
Temperature- and field-dependent Hall effect measurements are reported for
YbAgGe, a heavy fermion compound exhibiting a field-induced quantum phase
transition, and for two other closely related members of the RAgGe series: a
non-magnetic analogue, LuAgGe and a representative, ''good local moment'',
magnetic material, TmAgGe. Whereas the temperature dependent Hall coefficient
of YbAgGe shows behavior similar to what has been observed in a number of heavy
fermion compounds, the low temperature, field-dependent measurements reveal
well defined, sudden changes with applied field; in specific for a
clear local maximum that sharpens as temperature is reduced below 2 K and that
approaches a value of 45 kOe - a value that has been proposed as the
quantum critical point. Similar behavior was observed for where a
clear minimum in the field-dependent Hall resistivity was observed at low
temperatures. Although at our base temperatures it is difficult to distinguish
between the field-dependent behavior predicted for (i) diffraction off a
critical spin density wave or (ii) breakdown in the composite nature of the
heavy electron, for both field directions there is a distinct temperature
dependence of a feature that can clearly be associated with a field-induced
quantum critical point at persisting up to at least 2 K.Comment: revised versio
Modelling the incomplete Paschen-Back effect in the spectra of magnetic Ap stars
We present first results of a systematic investigation of the incomplete
Paschen-Back effect in magnetic Ap stars. A short overview of the theory is
followed by a demonstration of how level splittings and component strengths
change with magnetic field strength for some lines of special astrophysical
interest. Requirements are set out for a code which allows the calculation of
full Stokes spectra in the Paschen-Back regime and the behaviour of Stokes I
and V profiles of transitions in the multiplet 74 of FeII is discussed in some
detail. It is shown that the incomplete Paschen-Back effect can lead to
noticeable line shifts which strongly depend on total multiplet strength,
magnetic field strength and field direction. Ghost components (which violate
the normal selection rule on J) show up in strong magnetic fields but are
probably unobservable. Finally it is shown that measurements of the integrated
magnetic field modulus are not adversely affected by the Paschen-Back
effect, and that there is a potential problem in (magnetic) Doppler mapping if
lines in the Paschen-Back regime are treated in the Zeeman approximation.Comment: 8 pages, 10 figures, to appear in MNRA
Optical investigation of the metal-insulator transition in
We present a comprehensive optical study of the narrow gap
semiconductor. From the optical reflectivity, measured from the far infrared up
to the ultraviolet spectral range, we extract the complete absorption spectrum,
represented by the real part of the complex optical
conductivity. With decreasing temperature below 80 K, we find a progressive
depletion of below cm, the
semiconducting optical gap. The suppressed (Drude) spectral weight within the
gap is transferred at energies and also partially piles up over a
continuum of excitations extending in the spectral range between zero and
. Moreover, the interaction of one phonon mode with this continuum leads
to an asymmetric phonon shape. Even though several analogies between
and were claimed and a Kondo-insulator scenario was also invoked for
both systems, our data on differ in several aspects from those of
. The relevance of our findings with respect to the Kondo insulator
description will be addressed.Comment: 17 pages, 5 figure
Anisotropic optical conductivity of the putative Kondo insulator CeRuSn
Kondo insulators and in particular their non-cubic representatives have
remained poorly understood. Here we report on the development of an anisotropic
energy pseudogap in the tetragonal compound CeRuSn employing optical
reflectivity measurements in broad frequency and temperature ranges, and local
density approximation plus dynamical mean field theory calculations. The
calculations provide evidence for a Kondo insulator-like response within the
plane and a more metallic response along the c axis and qualitatively
reproduce the experimental observations, helping to identify their origin
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